A thermal type air flow meter capable of directly measuring a mass flow rate has been the mainstream of an air flow meter for detecting an amount of intake air in an internal combustion engine for use in, for example, an automobile.
In recent years, a technique has been proposed for manufacturing a sensor element of a thermal flow meter on a semiconductor substrate, such as silicon (Si), using micromachining technology. In such a sensor element of semiconductor type, a hollow portion is formed by removing part of the semiconductor substrate, the part having a rectangular shape. Then, an electrically insulating film having a thickness of several microns is formed on the hollow portion and a heating resistor is formed on the electrically insulating film. In addition, a temperature difference system that includes temperature sensors (temperature-sensitive resistors) disposed at points upstream and downstream of the heating resistor may be incorporated to determine the direction in which the fluid flows, whether a forward flow or a backward flow, based on the difference in temperature between the these points. The heating resistor is as small as having a size of several hundreds of microns and formed into a thin film. The heating resistor thus has a small heat capacity and is capable of exhibiting fast response and low electric power consumption.
Patent document 1 discloses a technique attempting to achieve low electric power consumption of the sensor element. The technique disclosed in patent document 1 attempts to achieve low electric power consumption while maintaining sensitivity by incorporating a heating resistor having a shorter length.